The present invention relates to a method for the oxidation of values which are present in low valence stages and are dissolved in liquids and to the oxidation of compounds which stabilize these low valence stages.
The selection of an oxidation agent suitable for the oxidation of values which are dissolved in liquids depends substantially on the type of values, on the type of solution in which the values are contained, and on other process parameters. If the oxidation of the values is a partial step, i.e., one step in a total process in which the values are to be separated or recovered, such as, for example, the oxidation of actinides in a reprocessing process for irradiated nuclear fuel and/or breeder materials, the process conditions which must be met to assure flawless operation and the most optimum sequence in the process will permit the use of only certain types of oxidation agents.
For example, aqueous reprocessing processes for irradiated nuclear fuel and/or breeder materials where the values uranium, plutonium and possibly other actinides are obtained in pure form with the aid of liquid-liquid extractions and subsequent further purification measures employ (1) salt-type oxidation agents, such as aqueous sodium nitrite solutions, or (2) gaseous oxidation agents, such as nitric oxides or nitric oxide mixtures (NO.sub.2 or N0+ NO.sub.2, respectively) together with transporting air, in order to oxidize U(IV) to U(VI) or Pu(III) to PU(IV), etc.
The salt-type oxidation agents and the gaseous oxidation agents each have drawbacks. Thus, a drawback in the processes which use salt-type oxidation agents, such as an aqueous sodium nitrite solution, is that the salt-type oxidation agents introduce foreign ions into the process liquid, and thus increase the proportion of solids in the waste material. The processes which use gaseous oxidation agents have the drawback that the gaseous oxidation agents, such as NO.sub.2 or a mixture of NO and NO.sub.2, react according to stoichiometrically-unfavorable gas-liquid reactions. Therefore, it is always necessary when using gaseous oxidation agents to have a great excess of oxidation agent which again results in heavy stresses on the ventilation systems. Moreover, the required transporting air for the gaseous oxidation agents also removes part of the gaseous oxidation agents from the reaction chamber without them being used.
It has also been attempted to use dinitrogen tetroxide (N.sub.2 O.sub.4) from commercially-available gas bombs. In this case, nitrogen dioxide (NO.sub.2) was produced by heating the N.sub.2 O.sub.4 and was conducted into the oxidation vessels in a stream of transporting air. When NO.sub.2 gas from the N.sub.2 O.sub.4 gas bombs was used, it was also necessary to have high excess quantities, e.g., about ten times the stoichiometric quantity. The amount of apparatus required for large throughputs is also considerable. The gasification of the liquids being treated must take place in trickling columns, and the excess of oxidation agents must be removed in a subsequent stripping column by blowing in air. This leads to large quantities of contaminated exhaust gas. The costs for the oxidation step with gaseous NO.sub.2 from N.sub.2 O.sub.4 are high. The costs merely for the NO.sub.2 gas from N.sub.2 O.sub.4 amount to about $250.00 per ton of light water reactor fuel to be reprocessed.